ColumnReader.loadVector should initiate CometDictionary after re-import arrays

[viirya]

Describe the bug

During debugging some test failures in #437, I found that CometScanExec returns empty dictionary values after first batch in failed queries. It is because in Data.importVector API call, it will release existing dictionary vector in the given dictionaryProvider and re-import the dictionary vector.

Currently we only initiate CometDictionary at the first batch and reuse it for later batches. Because of above issue, we should already initiate CometDictionary. It is simply re-importing existing dictionary vector allocated by native side, so it doesn't incur re-allocation of memory.

Steps to reproduce

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Expected behavior

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Additional context

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